1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) communication system and a channel estimating method used in the same and, more particularly to, a channel estimating method in a CDMA communication system.
The present application claims priority of Japanese Patent Application No.2000-144235 filed on May 17, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
Recently, a CDMA communication system resistant to interference and disturbance has been attracting the world attention as a communication system used in a mobile communication system. By the CDMA communication system, a user signal to be transmitted is diffused with a diffusion code and transmitted on a transmission side and, when received on a reception side, undergoes reverse diffusion by use of the same diffusion code, thus recovering the original user signal.
The following will describe a receiving method in the above-mentioned CDMA communication system with reference to FIG. 2. A base station 20 diffuses a user signal and multiplies thus diffused signal and a LOCAL signal to form a modulated RF signal and then transmits it from an antenna 21.
The radio wave transmitted from the base station 20 passes through a propagation path and arrives at a mobile station 10. The radio wave specifically enters the mobile station 10 through its antenna 11 and then is restored to a base band signal at a mixer 13 of an RF unit 12. The base band signal is converted into a digital signal by an ADC (Analog to Digital Converter) 14 and then restored to the user signal by a digital base band processing circuit 16 of a digital base band processing unit 15.
In this case, however, since the radio wave transmitted from the base station 20 reaches the mobile station 10 as reflected by, for example, a building when passing through the propagation path, at the mobile station 10 the radio wave has a phase changed from that thereof when it was transmitted from the base station 20, so that it cannot be restored as is to its original user signal. To recover a change of this shape change, channel estimation is carried out to thereby restore the original user signal.
As shown in FIG. 3, the digital base band signal processing unit 15 comprises a reverse diffusion unit 17 including a mixer 17a and a reverse diffuser 17b and a channel estimating unit 18, in such a configuration that a digital signal input to the digital base band signal processing unit 15 is converted by the reverse diffuser 17b into symbol data transmitted at the base station 20 and enters the channel estimating unit 18, where the influence thereof given by the propagation path is measured and then cancelled, to thereby restore the original user signal.
The following will describe this channel estimating method. Data transmitted from the base station 20 to the mobile station 10 comprises data of an individual channel being user data such as speech and data of a common pilot channel, symbols of which are all known.
The individual channel has different data for different users, while the common pilot channel transmits same information to all users. Channel estimation refers to utilizing known data being transmitted from the base station 20 to measure a phase change of data caused by the building or a like, thus restoring the user data.
Now, slot formats for the individual channel and the common pilot channel are shown in FIG. 4. The base station 20 transmits data for each radio frame (10 ms), which consists of 15 time slots (time slots 1-15) as shown in FIG. 4.
Also, the common pilot channel and the individual channel are transmitted simultaneously, which common pilot channel has 10 data items (A) in one time slot, which are all known. The individual channel has, for example, 20 data items in one time slot, 16 data items of which are data symbols such as speech and four data items of which are pilot symbols. Those four pilot symbols are different with different slots, with their pilot symbol pattern being known.
Channel estimation by use of an individual channel is described with reference to FIG. 5. When the mobile station 10 receives an individual channel, its pilot symbol is used to estimate a channel because only the pilot symbol is known of each individual channel.
Since, however, the pilot symbol is given only four in one slot, the pilot symbols before and behind the relevant data must be used to know a phase change in the data symbol of an individual channel. To estimate the channel by thus using the preceding and following pilot symbols, a phase change in one slot must be interpolated using these pilot symbols, thus complicating the processing.
The following will describe channel estimation by use of a common pilot channel with reference to FIG. 6. Since the data items of common pilot channels are all known, a mobile station 10 can know a change in phase of the data of a common pilot channel it received which change is caused by an influence of the building or the like, by comparing that data with a known common pilot pattern.
Also, since the symbol data items of common pilot channels are all known, a phase change in each data symbol of an individual channel can be measured without being interpolated in one slot, thus simplifying the processing.
The following will describe a transmission power for a common pilot channel and an individual channel transmitted by the base station 20 with reference to FIG. 7. The common pilot channel is provided to transmit common information to all the users and so its transmission power gradually decreases as the relative distance of the mobile station 10 with respect to the base station 20 increases, whereas the individual channel is provided to transmit different information to the different users and so the base station 20 controls the transmission power so that the information may be received with the same transmission power level no matter where the mobile station 10 is positioned with respect to the base station 20.
In FIG. 7, if the mobile station 10 moves, for example, from a location A to a location B in a cell C, the power for transmission to the mobile station 10 gradually decreases because the mobile station 10 goes farther away from the base station 20 as it goes toward the location B. However, the same transmission power can be maintained in reception of an individual channel even if the mobile station 10 moves from the location A to the location B.
The above-mentioned prior art channel estimating method suffers from a problem that the reception quality differs with different respective distances between the base station 20 and the mobile station 10 during channel estimation by use of the common pilot channel data.
Besides, although the reception quality is maintained at a constant level irrespective of the respective distance between the base station 20 and the mobile station 10 when a channel is estimated using the individual channel data, a phase change in one slot needs to be interpolated with a pilot symbol, thus problematically complicating the processing.
Further, as shown in FIG. 8, when a channel is estimated for both a common pilot channel and an individual channel using mixers 31 and 32, a common pilot channel-use reverse diffusion unit 33, an individual channel-use reverse diffusion unit 34, a common pilot channel-use channel estimating unit 35, and an individual channel-use channel estimating unit 36, the respective channel estimate results must be averaged at a channel estimate averaging unit 38 to thereby restore user data, thus problematically complicating the processing and increasing the current dissipation. A reference numeral 37 in FIG. 8 indicates a power calculating unit.